text: PCBs are derived from biphenyl, which has the formula CH, sometimes written (CH). In PCBs, some of the hydrogen atoms in biphenyl are replaced by chlorine atoms. There are 209 different chemical compounds in which one to ten chlorine atoms can replace hydrogen atoms. PCBs are typically used as mixtures of compounds and are given the single identifying CAS number . About 130 different individual PCBs are found in commercial PCB products.
  Toxic effects vary depending on the specific PCB. In terms of their structure and toxicity, PCBs fall into two distinct categories, referred to as coplanar or non-ortho-substituted arene substitution patterns and noncoplanar or ortho-substituted congeners.
  ;Coplanar or non-ortho
  :The coplanar group members have a fairly rigid structure, with their two phenyl rings in the same plane. It renders their structure similar to polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans, and allows them to act like PCDDs, as an agonist of the aryl hydrocarbon receptor (AhR) in organisms. They are considered as contributors to overall dioxin toxicity, and the term dioxins and dioxin-like compounds is often used interchangeably when the environmental and toxic impact of these compounds is considered.
  ;Noncoplanar
  :Noncoplanar PCBs, with chlorine atoms at the ortho positions can cause neurotoxic and immunotoxic effects, but only at concentrations much higher than those normally associated with dioxins. However, as they are typically found at much higher levels in biological and environmental samples they also pose health concerns, particularly to developing animals (including humans). As they do not activate the AhR, they are not considered part of the dioxin group. Because of their lower overt toxicity, they have typically been of lesser concern to regulatory bodies.
  Di-ortho-substituted, non-coplanar PCBs interfere with intracellular signal transduction dependent on calcium which may lead to neurotoxicity. ortho-PCBs can disrupt thyroid hormone transport by binding to transthyretin.